The present invention relates to a liquid crystal display device, and more specifically, to a liquid crystal display device of an alignment-divided vertical alignment type that permits display of a high contrast ratio and has a wide viewing angle characteristic.
In recent years, liquid crystal display devices (LCDs) have increasingly come into widespread use. Among various types of LCDs, mainstream has been a TN LCD in which a nematic liquid crystal material having positive dielectric anisotropy is twisted. The TN LCD however has a problem of being large in visual angle dependence that results from the alignment of liquid crystal molecules.
To improve the visual angle dependence, alignment-divided vertical alignment LCDs have been developed, and use of these LCDs is expanding. For example, Japanese Patent Publication No. 2947350 (Literature 1) discloses an MVA LCD as one of the alignment-divided vertical alignment LCDS. The MVA LCD, which includes a vertical alignment liquid crystal layer placed between a pair of electrodes to present display in the normally black (NB) mode, is provided with domain regulating means (for example, slits and/or protrusions) to enable liquid crystal molecules in each pixel to fall (tilt) in a plurality of different directions during application of a voltage.
Recently, needs for displaying moving image information have rapidly increased, not only in LCD TVs, but also in PC monitors and portable terminal equipment (such as mobile phones and PDAs). To display moving images with high definition on LCDs, it is necessary to shorten the response time (increase the response speed) of the liquid crystal layer, so that a predetermined grayscale level can be reached within one vertical scanning period (typically, one frame).
As one method for improving the response characteristic of an MVA LCD, it is conceivable to increase the entire length of ribs placed in each pixel or increase the height of such ribs. That is, by increasing the entire area of slope faces of ribs that are in contact with the liquid crystal layer, the alignment regulating force against the liquid crystal layer can be enhanced, and this improves the response characteristic (Literature 1).
Increasing the area of slope faces of ribs however reduces the contrast ratio because the luminance in the black display state (called “black luminance” in some cases) increases near slope faces of ribs, as described in Literature 1 (see FIGS. 28 to 32 of this literature, for example). Literature 1 describes that as a result of evaluation of LCDs having 0.7 μm to 2.0 μm high ribs placed in both surface portions of the liquid crystal layer, the reduction in contrast ratio was of a level so low that no problem would be presented in actual observation.
However, according to examinations by the present inventors, the area of slope faces of ribs is a bit factor that substantially determines the black luminance, and thus it is not easy to improve the response characteristic while securing a contrast ratio of a given level (700:1 or higher, for example).
In view of the above, a main object of the present invention is providing an alignment-divided vertical alignment LCD that can improve its response characteristic while suppressing reduction in contrast ratio.